The phenothiazine group of drugs including thioridazine has been widely prescribed for the treatment of psychiatric disorders since the 1940s. The anti-cancer activities of these antipsychotics have intrigued clinicians and researchers since the early 1970s. Whereas thioridazine and several other phenothiazines can block the cardiac hERG voltage-gated potassium channels, they remain on the market because their risk for prolonging the cardiac QTc interval is outweighed by their beneficial effects. The possibility that potassium channel block is one molecular mechanism by which these antipsychotics protect against cancer has never been considered before. We propose to test the original hypothesis that thioridazine protects against medulloblastoma (MB) growth and metastasis by blocking the EAG2 voltage-gated potassium channels that are upregulated in a subset of MBs of human patients, particularly in metastatic MBs. Specifically, we hypothesize that (1) thioridazine block of EAG2 channels prevents MB cell volume reduction for premitotic condensation (PMC) so as to cause cell cycle arrest - those MB cells that venture beyond the G2 phase encounter mitotic catastrophe and perish via apoptosis, and (2) thioridazine block of EAG2 channels reduces water efflux from the trailing edge of migrating MB cells, thereby interfering with MB cell migration by preventing the trailing edge of the cell from shrinking - a local volume regulation essential for cell movement. To test our hypothesis that thioridazine block of EAG2 potassium channels that appear on the surface of mitotic MB cells and on the trailing edge of migrating MB cells to protect against MB growth and metastasis, we will conduct in vitro and in vivo studies to experimentally validate the prediction that the anti-cancer activites of thioridazine can be mimicked by pharmacological treatment with astemizole, a structurally unrelated EAG2 channel blocker. We will further test whether the anti-cancer activities of these two EAG2 channel blockers resemble the effects of reducing EAG2 expression of human MB cells in vitro and in vivo - for mice bearing human MB xenograft, as well as the effects of knocking out Eag2 in mouse MB models. Additional controls will be conducted to confirm that the anti-cancer activities of thioridazine that arise from its block of EAG2 channels are occluded by shRNA knockdown of EAG2 or genetic deletion of Eag2 in mouse models.